Kinetics of CN reactions with N2O and CO2

✍ Scribed by N. S. Wang; D. L. Yang; M. C. Lin; C. F. Melius

• Publisher: John Wiley and Sons
• Year: 1991
• Tongue: English
• Weight: 493 KB
• Volume: 23
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550230206

✦ Synopsis

The rate constants for the reaction of CN with NzO and COz have been measured by the laser dissociation/laser-induced fluorescence (two-laser pump-probe) technique at temperatures between 300 and 740 K. The rate of CN + NzO was measurable above 500 K, with a least-squares averaged rate constant, k = 10-'18'04 exp(-3560 f 181/T) cm3/s. The rate of CN + COz, however, was not measurable even at the highest temperature reached in the present work, 743 K, with [COz] 5 1.9 x 10'' molecules/cm3.

In order to rationalize the observed kinetics, quantum mechanical calculations based on the BAC-MP4 method were performed. The results of these calculations reveal that the CN + NZ0 reaction takes place via a stable adduct NCNNO with a small barrier of 1.1 kcal/mol. The adduct, which is more stable than the reactants by 13 kcal/mol, decomposes into the NCN i NO products with an activation energy of 20.0 kcal/mol. This latter process is thus the ratecontrolling step in the CN + NzO reaction. The CN + COz reaction, on the other hand, occurs with a large barrier of 27.4 kcal/mol, producing an unstable adduct NCOCO which fragments into NCO + CO with a small barrier of 4.5 kcal/mol. The large overall activation energy for this process explains the negligibly low reactivity of the CN radical toward COZ below 1000 K.

Least-squares analyses of the computed rate constants for these two CN reactions, which fit well with experimental data, give rise to = 6.4 x T26 exp(-1860/T) cm3/s kcoz = 6.1 x lo-'' T Z 2 exp(-13530/T) cm3/s for the temperature range 300-3000 K.